The developmental program of the cellular slime mold Dictyostelium discoideum is regulated by at least two extracellular signals: cAMP pulses and the formation of cellular aggregates. Both of these singals induce the accumulation of new mRNA species, and both are required for the successful completion of the developmental program. The goal of the proposed experiments is to investigate the mechanisms by which one of these signals, extracellular cAMP, modulates gene expression. This process consists of at least three separate parts: (1) detection of extracellular cAMP, (2) transmission of a signal through the cell membrane and cytoplasm, most likely by a "second messenger", and (3) interaction of that intracellular signal, or some molecule modified as a result of the signal, with cAMP inducible genes to modify transcription, and/or mRNA synthesized from such genes to alter their accumulation. By initiating characterization of each of these aspects of cAMP signal transduction, it is hoped that a basic outline of the pathway can be established. Detection of extracellular cAMP most likely occurs through specific cell surface receptors. Identification, purification and structural characterization of the receptor, facilitated by molecular cloning of cDNAs encoding the receptor, are proposed. The correlation of intracellular changes known to be induced by cAMP with induction of cAMP regulated genes is proposed to focus future investigation of the intracellular second messenger involved in transmission of the signal between the cell surface and the gene. Finally, structural studies of cAMP induced genes, leading to the eventual identification of specific regulatory sequences required for cAMP gene induction, and initial studies to identify potential regulatory proteins that specifically interact with these sequences will initiate characterization of the "target" of the cAMP signal transduction process. The question of how extracellular signal modulate gene expression is a general one. Developing cells are constantly exposed to extracellular signals, be they hormones, nutrients or specific interactions between cells or between a cell and the extracellular matrix. Characterization of a relatively simple signal transduction path may contribute to our understanding of the mechanisms by which mammalian development is regulated.